Ant colonies use scent-based 'birth control'

If a worker ant dares to reproduce in the presence of the queen, her sisters will smell her attempt and attack, according to a new study.

Typically, only queens produce offspring in an ant colony, and males die after mating. The sons and the daughter queens fly away, with hopes of reproducing elsewhere, while the worker daughters stay on to build the colony and care for the next generation.

These worker ants are biologically capable of a type of parthenogenesis, the process that allows a female to produce offspring without a mate. When they try, however, they produce chemicals called pheromones that their sisters detect with antennas.

“It's basically smell, but not the smell we know,” said study co-author Jurgen Liebig of Arizona State University.

If the colony lacks a queen, workers are permitted to have their own babies, Liebig explained. But when a queen is present, only she is allowed to produce the pheromone that signals fertility status. If a worker tries to “cheat,” her sisters will physically restrain the disobedient ant from successfully reproducing.

The research was published online in the journal Current Biology.

Previous studies showed a correlation between ants' reproductive policing behavior and these pheromones, Liebig said, so there was strong reason to believe the chemicals were tipping them off.

“The problem was that nobody could ever show it,” he said.

Liebig's team studied the ant species Aphaenogaster cockerelli because it uses a simple version of the compound that the scientists could easily obtain.

When graduate student Adrian Smith, the study's lead author, applied the compound to worker ants in naturally occurring amounts, the ants were attacked by their sisters.

“When we used a different (chemical) that is not specific to these reproductive cheaters, there was no effect,” Liebig said. “So it really was the specific compound that identifies them as reproductive cheaters.”

Les Greenberg, of the University of California Riverside, said the ability to manipulate ants' chemical signals artificially should also help unravel other types of insect communication.

“The study is a fascinating example of how social insects maintain order in their societies,” said Greenberg, who was not involved with the research. “The analysis of other behaviors, such as dominance hierarchies and nest-mate recognition, should be possible with these techniques.”

Liebig believes comparisons can be drawn to some extent between ant colony behavior and human social interaction.

“I think it's a characteristic of any society that there's some temptation to cheat,” he said. “In ants we have these same problems: We have colony interests and individual interests. . . . If everybody just follows his own interest, the benefit of cooperation would no longer be there.”

But Mark Deyrup, a senior research biologist at Archbold Biological Station in Florida, questioned whether the findings can be applied so neatly to human social behavior.

“In human societies, those who take on the roles and privileges of a poorly functioning anointed leader may actually promote societal stability,” said Deyrup, who was not involved in the study.

Still, Deyrup described the new research as “fascinating” and “particularly commendable for its meticulous tracing of the pathways of cause and effect.”

Deyrup, Greenberg and Liebig all believe fertility-status pheromones could play similar roles in other social insects, possibly including some bees and wasps.